The present invention relates to a driving method and driving device of a liquid crystal panel having a switching element, for example, such as a Thin Film Transistor (TFT), and in particular to the driving method and driving device of a liquid crystal panel which compensates for the shortage of charges supplied to respective pixels of the liquid crystal panel.
In recent years, a liquid crystal display device adopting a TFT (hereinafter referred to as TFT-LCD) has been increased in size and become higher definition, and there is such a tendency that a demand for higher display quality of the TFT-LCD has been increased as the TFT-LCD is developed with multimedia.
In such a development of the TFT-LCD which has been increased in size and become higher definition, problems such as shortened time for writing a picture signal onto a pixel and a serious wiring delay due to respective buses have been noted.
Here, driving methods of the TFT-LCD can roughly be divided into two methods: line reversal driving and dot reversal driving.
The line reversal driving refers to driving which reverses a polarity of a signal line in one to several horizontal time (=about 16.7 mS/number of scanning lines), which has the characteristic of resisting crosstalk in a vertical direction (crosstalk in the vertical direction occurs less often). In addition, since a voltage is applied to a liquid crystal by synthesizing a common electrode potential and a signal line potential, the line reversal driving has such an advantage that the voltage of not more than a threshold value of the liquid crystal can be obtained from the common electrode potential, and thereby an amplitude of the signal line potential is required to have as low a voltage as a dynamic range Vdy of the liquid crystal.
On the other hand, the dot reversal driving refers to driving which reverses the polarity of the signal line in one to several horizontal time (=about 16.7 mS/number of scanning lines) while also reversing a polarity of an adjacent signal line, which has the characteristic of resisting crosstalk in both of the vertical and horizontal directions. Despite its superior display quality, however, this driving method requires that the common electrode potential be constant, and thus has such a drawback that the amplitude of the signal line potential needs to have a high voltage of 2xc3x97(Vth+Vdy), where Vth is the threshold value of the liquid crystal.
Meanwhile, the difference in amplitude of the signal line potentials in the line and dot reversal driving corresponds to difference in endurance of their source drivers, which results in difference in driver cost between the two driving methods. Therefore, when attempting to provide a low-cost and high-quality TFT-LCD, it is more effective to compose the TFT-LCD by using a driver which conducts the line reversal driving in which the signal line potential is more voltage-saving compared with the dot reversal driving.
However, in the line reversal driving, a charge supplying ability of a common electrode line which supplies respective pixels of the TFT-LCD with charges becomes deficient at early stages of writing. Here, the common electrode line is provided parallel to a scanning line which is provided on a pixel substrate, on a counter substrate opposite to the pixel substrate on which the TFT is provided. FIG. 6 shows an equivalent circuit of the common electrode line. This equivalent circuit is, though having inputs at both ends, basically made up of an integration circuit (low-pass filter) of FIG. 7. When a rectangular wave is inputted to this circuit, as shown in FIG. 7, an output waveform thereof grows dull due to a characteristic of the integration circuit, i.e. this means the shortage of the charge supplying ability at the early stages of writing. Note that, in FIG. 6, the waveform grows duller as it moves from the both ends toward the center.
When the charge supplying ability of the common electrode line becomes thus insufficient, crosstalk appears in the horizontal direction, i.e. the direction parallel to the scanning line. This crosstalk in the horizontal direction, as shown in FIG. 8, appears particularly when, for example, displaying a black square 52 at the center of a screen having a background of a half tone part 51 (part of the screen indicated by vertical hatching). More specifically, the sides of the black square 52 become whiter than the half tone part 51.
Here, by forming a thick common electrode line for example, crosstalk in the horizontal direction can be settled on the side of a panel. However, this method results in reducing a numerical aperture, thereby being less preferable.
Accordingly, for example, Japanese Unexamined Patent Publication No. 191821/1992 (Tokukaihei 4-191821 published on Jul. 10, 1992) discloses a technique of reducing crosstalk in the horizontal direction without reducing the numerical aperture of the panel by incorporating a panel impedance which is connected to the common electrode of the TFT-LCD into a negative feedback circuit adopting an operational amplifier. The structure of the circuit according to the publication is shown in FIG. 9.
In this driving circuit, an original input (signal inputted to the operational amplifier OP) is amplified by a value obtained by dividing a synthetic impedance which is composed of resistors R1 and R2, a capacitor C, and a panel impedance of a liquid crystal panel 55, by a resistor R3. Utilizing the characteristic of the negative feedback of the operational amplifier OP, a voltage which is determined in anticipation of a reduction in voltage inside of the panel is inputted to the common electrode line of the liquid crystal panel 55, thereby reducing crosstalk in the horizontal direction. The original input, waveforms of the output and input of the liquid crystal panel 55 at that time are shown in FIGS. 10(a) through 10(c).
Note that, a resistor R4 which is connected to a ground input of the operational amplifier OP is to determine an offset voltage of the counter electrode of the liquid crystal panel 55. In addition, respective values (circuit constants) of the resistors R1, R2, R3 and R4, and the capacitor C, etc., need to be optimized.
However, a problem arises when the charge supplying ability of the common electrode line is considerably low, i.e. the driving above cannot fully settle crosstalk in the horizontal direction. The reason is that, when the charge supplying ability of the common electrode line is considerably low, the degree of crosstalk in the horizontal direction becomes different between in the vicinity of the black square and in the vicinity of the input when the black square 52 appears in the middle of the screen having the background of the half tone part 51.
Thus, as shown in FIG. 11, when, for example, the circuit constants of the negative feedback circuit are set so that crosstalk disappears in the vicinity of the input, crosstalk in the horizontal direction still remains in the vicinity of the black square. On the other hand, as shown in FIG. 12, when, for example, the circuit constants of the negative feedback circuit are set so that crosstalk disappears in the vicinity of the black square, the crosstalk in the horizontal direction in the vicinity of the input appears in a different form which is blacker than the background of the half tone part 51. Note that, it has been known that even if, for example, the circuit constants of the negative feedback circuit are set so that crosstalk at an arbitrary position between the vicinities of the input and black square disappears, different types of crosstalk appear in the vicinities of the input and black square.
Note that, in FIGS. 11 and 12, hatching density (interval between vertical lines) shows the thickness of the half tone in qualitative terms. Namely, the denser the hatching is, the thicker the half tone is, while the sparser the hatching is, the thinner the half tone is.
The reason for thus resulting in different degrees of crosstalk in the horizontal direction is as follows: when the charge supplying ability of the common electrode line is considerably low, that is, when a time constant CR which is determined by a wiring resistor R and load capacitance (load capability) C of the common electrode line is too large, and a desired voltage is not reached within desired writing time, a charging quantity of pixels reacts sensitively with respect to fluctuation in the time constant CR. R is constant when determining the time constant CR. However, the time constant CR fluctuates as C fluctuates depending on the content of display.
Incidentally, even when the time constant CR is at its maximum (i.e. when displaying entirely black display in a normally white mode), the charging quantity of pixels can remain constant regardless of what contents are displayed on the screen, provided that a desired voltage has been attained within desired writing time. In this case, crosstalk does not occur in the horizontal direction.
It is an object of the present invention to provide a driving method and driving device of a liquid crystal panel capable of preventing crosstalk having different degrees depending on a location while increasing display quality even when a charge supplying ability with respect to each pixel of the liquid crystal panel is considerably low.
In order to attain the foregoing object, in the driving method of the liquid crystal panel according to the present invention, in a driving method adopting a driving signal which periodically drives respective pixels of the liquid crystal panel, the driving signal includes a compensating signal for compensating for a deficiency in charges with which the respective pixels are supplied at the beginning of respective driving which is repeated periodically.
In the foregoing structure, since the driving signal includes the compensating signal, a quantity of charges in each pixel which is deficient at the beginning of respective driving which is repeated periodically, i.e. at early stages of writing, is compensated by the compensating signal, regardless of whether it is large or small. Consequently, a desired charging quantity can be obtained in each pixel within desired writing time, and when, for example, performing black display in the middle of a screen having a half-tone background, the charging quantity of the respective pixels can be made constant at the both ends of the black display at the early stages of writing, thereby surely preventing crosstalk having different degrees depending on a location due to difference in a charging quantity while surely improving display quality.
Further, in order to attain the foregoing object, the driving device of the liquid crystal panel according to the present invention, in a driving device of the liquid crystal panel in which each pixel of the liquid crystal panel is driven by the driving signal, includes an adder circuit for generating the driving signal by addition of a first wave and a second wave, the first wave being a rectangular wave to be a base of the driving signal, the second wave capable of increasing respective amplitudes of the first wave in rising and falling directions when the first wave rises and falls, respectively.
In the foregoing structure, by addition of the first wave of the rectangular wave which becomes the base of the driving signal and the second wave (e.g. a rectangular wave or a sinusoidal wave) performed by the adder circuit, the amplitudes of the first wave in the rising and falling directions increase compared with those in a state before the addition of the second wave. Note that, the foregoing addition may be performed by various methods such as inversion addition or addition of subtracted wave. By adopting a signal after addition as the driving signal of the liquid crystal panel, a quantity of charges with which respective pixels of the liquid crystal panel are supplied at the early stages of writing surely increases compared with that before the addition.
Consequently, even when a time constant of wiring for supplying charges is increased with the increasing size of the liquid crystal panel while a quantity of charges in shortage in respective pixels due to wiring delay becomes considerably large, the use of the foregoing driving signal enables the respective pixels to surely obtain a desired charging quantity within desired writing time. Consequently, when, for example, performing black display in the middle of a screen having a half-tone background, the charging quantity of the respective pixels can be made constant at the both ends of the black display, thereby surely preventing crosstalk having different degrees depending on a location due to difference in a charging quantity while surely improving display quality.
Additional objects, features, and strengths of the present invention will be made clear by the description below. Further, the advantages of the present invention will be evident from the following explanation in reference to the drawings.